Scientists explore causes of the pause in warming, perhaps the most important research of the decade

Summary: Today we look at papers exploring the causes of the pause in warming of the surface atmosphere temperature, now {June 2014} in its second decade. This is one of the world’s most important science priorities, which will guide public policy for the next decade. Do we need a crash program to control CO2 emissions, and mitigate the effects of warming. Or do we have time for more measured responses? There is another dimension to this. Climate science not only teaches us much about our planet, but the reactions of political activists on both says much about modern America.

“The biggest mystery in climate science today…”
— “Climate change: The case of the missing heat“, Jeff Tollefson, Nature, 15 January 2014 — “Sixteen years into the mysterious ‘global-warming hiatus’, scientists are piecing together an explanation.”

“Overdetermination is a phenomenon whereby a single observed effect is determined by multiple causes at once, any one of which alone might be enough to account for (‘determine’) the effect. That is, there are more causes present than are necessary to cause the effect.” (From Wikipedia}

Content

Updated – there are almost 100 papers cited, grouped by cause of the pause.

Introduction

Surveys of the literature

Less water vapor in the atmosphere

Natural variability

More aerosols (e.g., burning coal)

Volcanoes

Changes in oceans & ocean-air cycles

“Stadium Waves”

It’s the reduced emissions of CFCs and methane

The Sun

A temperature dataset mismeasures warming

Winds

Clouds

Multiple causes, other causes, or unknown causes

Conferences

For More Information

(1) Introduction

Research into the causes of the pause in surface temperature warming is some of the most important science being done today. We will gain not only vital information about our world, but also — if we look — insights about ourselves that can help us become a more effective society.

(a) Learning about our world

The results of this research might determine public policy priorities for the next decade or more. Do we have time for measured responses to rising CO2 levels, or are crash programs required now to limit them — and prepare for the effects of resumed warming?

Here are some samples of papers about possible causes of the pause. These are all speculative (science takes place on the edges of available data and theory). Eventually these lines of research will produce answers. Meanwhile we get to watch science in action.

(b) Learning about ourselves

American society has long been famously anti-intellectual. The Right cherishes creationists; the Left loves the Pope when he supports their views (He’s a superstitious old reactionary the rest of the time). The climate wars demonstrates that this remains true today.

Much of the lay debate on both sides consists of personal attacks on climate scientists (guilty of playing for the wrong team). Skeptic websites overflow with attacks on science.

Climate activist website have abandoned the IPCC as insufficiently alarmist, relying largely on exaggerations of conclusions of outlier papers in the climate literature to paint doomster scenarios as inevitable (the IPCC being created to discourage this).

What’s more pitiful? Activists attempts to conceal the work of climate scientists? Or their followers who close their eyes, seeing only what they’re told to see (a modern version of the prisoners in Plato’s cave).

Now — onto the science. This is a brief summary of research conducted about climate by scientists in many fields. There are almost a hundred papers, grouped in the crude 12 kinds of explanation of the pause — shown in chronological order. This will be updated as new papers appear. These are given as examples of work on this exciting frontier of climate science. This is not a comprehensive bibliography, nor does it attempt to list all the major papers in each of these areas.

(2) Surveys of the literature

Here are survey papers, reviewing the current state of the various explanations of the pause.

There are two main ways to explain the recent surface temperature behaviour; firstly, through changes in the net amount of incoming energy to the climate system (radiative forcing) or, secondly, through redistribution of energy within the climate system, particularly through exchange between the upper and deep ocean, which can temporarily hide the warming below the surface.

(b) “Heat hide and seek“, Lisa Goddard, Nature Climate Change, March 2014 — “Natural variability can explain fluctuations in surface temperatures but can it account for the current slowdown in warming?”

Climate models projected stronger warming over the past 15 years than has been seen in observations. Conspiring factors of errors in volcanic and solar inputs, representations of aerosols, and El Niño evolution, may explain most of the discrepancy.

Conclusion (red emphasis added

We conclude that use of the latest information on external influences on the climate system and adjusting for internal variability associated with ENSO can almost completely reconcile the trends in global mean surface temperature in CMIP5 models and observations. Nevertheless, attributing climate trends over relatively short periods, such as 10 to 15 years, will always be problematic, and it is inherently unsatisfying to find model–data agreement only with the benefit of hindsight. We see no indication, however, that transient climate response is systematically overestimated in the CMIP5 climate models as has been speculated, or that decadal variability across the ensemble of models is systematically underestimated, although at least some individual models probably fall short in this respect.

Most importantly, our analysis implies that significant warming trends are likely to resume, because the dominant long-term warming effect of well-mixed greenhouse gases continues to rise. Asian pollution levels are likely to stabilize and perhaps decrease, although lower solar activity may persist and volcanic eruptions are unpredictable. ENSO will eventually move back into a positive phase and the simultaneous coincidence of multiple cooling effects will cease. Further warming is very likely to be the result.

(3) Reduced level of water vapor in the atmosphere

Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000–2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% as compared to estimates neglecting this change. These findings show that stratospheric water vapor is an important driver of decadal global surface climate change.

Excerpt:

Over the past century, global average surface temperatures have warmed by about 0.75°C. Much of the warming occurred in the past half-century, over which the average decadal rate of change was about 0.13°C, largely due to anthropogenic increases in well-mixed greenhouse gases. However, the trend in global surface temperatures has been nearly flat since the late 1990s despite continuing increases in the forcing due to the sum of the well-mixed greenhouse gases (CO2, CH4, halocarbons, and N2O), raising questions regarding the understanding of forced climate change, its drivers, the parameters that define natural internal variability, and how fully these terms are represented in climate models.

… Figure 3 thus shows that the decline in stratospheric water vapor after 2000 should be expected to have significantly contributed to the flattening of the global warming trend in the past decade, and stratospheric water increases may also have acted to steepen the observed warming trend in the 1990s.

Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings. Declining solar insolation as part of a normal eleven-year cycle, and a cyclical change from an El Nino to a La Nina dominate our measure of anthropogenic effects because rapid growth in short-lived sulfur emissions partially offsets rising greenhouse gas concentrations.

As such, we find that recent global temperature records are consistent with the existing understanding of the relationship among global surface temperature, internal variability, and radiative forcing, which includes anthropogenic factors with well known warming and cooling effects.

We analyze five prominent time series of global temperature (over land and ocean) for their common time interval since 1979: three surface temperature records (from NASA/GISS, NOAA/NCDC and HadCRU) and two lower-troposphere (LT) temperature records based on satellite microwave sensors (from RSS and UAH). All five series show consistent global warming trends ranging from 0.014 to 0.018 K yr−1.

When the data are adjusted to remove the estimated impact of known factors on short-term temperature variations (El Niño/southern oscillation, volcanic aerosols and solar variability), the global warming signal becomes even more evident as noise is reduced. Lower-troposphere temperature responds more strongly to El Niño/southern oscillation and to volcanic forcing than surface temperature data. The adjusted data show warming at very similar rates to the unadjusted data, with smaller probable errors, and the warming rate is steady over the whole time interval. In all adjusted series, the two hottest years are 2009 and 2010.

An approach complementary to General Circulation Models (GCMs), using the anthropogenic CO2 radiative forcing as a linear surrogate for all anthropogenic forcings, was recently developed for quantifying human impacts. Using preindustrial multiproxy series and scaling arguments, the probabilities of natural fluctuations at time lags up to 125 years were determined. The hypothesis that the industrial epoch warming was a giant natural fluctuation was rejected with 99.9% confidence. In this paper, this method is extended to the determination of event return times. Over the period 1880–2013, the largest 32 year event is expected to be 0.47 K, effectively explaining the postwar cooling (amplitude 0.42–0.47 K). Similarly, the “pause” since 1998 (0.28–0.37 K) has a return period of 20–50 years (not so unusual). It is nearly cancelled by the pre-pause warming event (1992–1998, return period 30–40 years); the pause is no more than natural variability.

The observed global-warming rate has been nonuniform, and the cause of each episode of slowing in the expected warming rate is the subject of intense debate. To explain this, nonrecurrent events have commonly been invoked for each episode separately. After reviewing evidence in both the latest global data (HadCRUT4) and the longest instrumental record, Central England Temperature, a revised picture is emerging that gives a consistent attribution for each multidecadal episode of warming and cooling in recent history, and suggests that the anthropogenic global warming trends might have been overestimated by a factor of two in the second half of the 20th century.

A recurrent multidecadal oscillation is found to extend to the preindustrial era in the 353-y Central England Temperature and is likely an internal variability related to the Atlantic Multidecadal Oscillation (AMO), possibly caused by the thermohaline circulation variability. The perspective of a long record helps in quantifying the contribution from internal variability, especially one with a period so long that it is often confused with secular trends in shorter records. Solar contribution is found to be minimal for the second half of the 20th century and less than 10% for the first half.

The underlying net anthropogenic warming rate in the industrial era is found to have been steady since 1910 at 0.07–0.08 °C/decade, with superimposed AMO-related ups and downs that included the early 20th century warming, the cooling of the 1960s and 1970s, the accelerated warming of the 1980s and 1990s, and the recent slowing of the warming rates. Quantitatively, the recurrent multidecadal internal variability, often underestimated in attribution studies, accounts for 40% of the observed recent 50-y warming trend.

Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century, challenging the prevailing view that anthropogenic forcing causes climate warming.

Various mechanisms have been proposed for this hiatus in global warming, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model.

Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase.

An approach complementary to General Circulation Models (GCMs), using the anthropogenic CO2 radiative forcing as a linear surrogate for all anthropogenic forcings [Lovejoy, 2014], was recently developed for quantifying human impacts. Using preindustrial multiproxy series and scaling arguments, the probabilities of natural fluctuations at time lags up to 125 years were determined. The hypothesis that the industrial epoch warming was a giant natural fluctuation was rejected with 99.9% confidence.

In this paper, this method is extended to the determination of event return times. Over the period 1880–2013, the largest 32 year event is expected to be 0.47 K, effectively explaining the postwar cooling (amplitude 0.42–0.47 K). Similarly, the “pause” since 1998 (0.28–0.37 K) has a return period of 20–50 years (not so unusual). It is nearly cancelled by the pre-pause warming event (1992–1998, return period 30–40 years); the pause is no more than natural variability.

Factors involved in the recent pause in the rise of global mean temperatures are examined seasonally. For 1999 to 2012, the hiatus in surface warming is mainly evident in the central and eastern Pacific. It is manifested as strong anomalous easterly trade winds, distinctive sea-level pressure patterns, and large rainfall anomalies in the Pacific, which resemble the Pacific Decadal Oscillation (PDO).

These features are accompanied by upper tropospheric teleconnection wave patterns that extend throughout the Pacific, to polar regions, and into the Atlantic. The extratropical features are particularly strong during winter. By using an idealized heating to force a comprehensive atmospheric model, the large negative anomalous latent heating associated with the observed deficit in central tropical Pacific rainfall is shown to be mainly responsible for the global quasi-stationary waves in the upper troposphere. The wave patterns in turn created persistent regional climate anomalies, increasing the odds of cold winters in Europe.

Hence, tropical Pacific forcing of the atmosphere such as that associated with a negative phase of the PDO produces many of the pronounced atmospheric circulation anomalies observed globally during the hiatus.

Most present-generation climate models simulate an increase in global-mean surface temperature (GMST) since 1998, whereas observations suggest a warming hiatus. It is unclear to what extent this mismatch is caused by incorrect model forcing, by incorrect model response to forcing or by random factors.

Here we analyse simulations and observations of GMST from 1900 to 2012, and show that the distribution of simulated 15-year trends shows no systematic bias against the observations. Using a multiple regression approach that is physically motivated by surface energy balance, we isolate the impact of radiative forcing, climate feedback and ocean heat uptake on GMST—with the regression residual interpreted as internal variability—and assess all possible 15- and 62-year trends. The differences between simulated and observed trends are dominated by random internal variability over the shorter timescale and by variations in the radiative forcings used to drive models over the longer timescale. For either trend length, spread in simulated climate feedback leaves no traceable imprint on GMST trends or, consequently, on the difference between simulations and observations.

The claim that climate models systematically overestimate the response to radiative forcing from increasing greenhouse gas concentrations therefore seems to be unfounded.

The observed global mean surface air temperature (GMST) has not risen over the last 15 years, spurring outbreaks of skepticism regarding the nature of global warming and challenging the upper range transient response of the current-generation global climate models.Recent numerical studies have, however, tempered the relevance of the observed pause in global warming by highlighting the key role of tropical Pacific internal variability.

Here we first show that many climate models overestimate the influence of the El Niño–Southern Oscillation on GMST, thereby shedding doubt on their ability to capture the tropical Pacific contribution to the hiatus. Moreover, we highlight that model results can be quite sensitive to the experimental design. We argue that overriding the surface wind stress is more suitable than nudging the sea surface temperature for controlling the tropical Pacific ocean heat uptake and, thereby, the multidecadal variability of GMST.

Using the former technique, our model captures several aspects of the recent climate evolution, including the weaker slowdown of global warming over land and the transition toward a negative phase of the Pacific Decadal Oscillation. Yet the observed global warming is still overestimated not only over the recent 1998–2012 hiatus period but also over former decades, thereby suggesting that the model might be too sensitive to the prescribed radiative forcings.

Excerpt:

According to the technical summary of the latest Intergovernmental Panel on Climate Change Fifth Assessment Report (AR5), the observed recent warming hiatus, defined as the reduction in global mean surface temperature (GMST) trend during 1998–2012 as compared to the trend during 1951–2012, is attributed in roughly equal measures to a cooling contribution from internal variability and a reduced trend in external radiative forcing. This medium confidence expert judgement was, however, based on a small number of studies and has been recently contradicted by several numerical experiments suggesting that the hiatus can be fully explained by the internal multidecadal variability of the tropical Pacific Ocean …

The recent slowdown in global warming has brought into question the reliability of climate model projections of future temperature change and has led to a vigorous debate over whether this slowdown is the result of naturally occurring, internal variability or forcing external to Earth’s climate system. To address these issues, we applied a semi-empirical approach that combines climate observations and model simulations to estimate Atlantic- and Pacific-based internal multidecadal variability (termed “AMO” and “PMO,” respectively). Using this method, the AMO and PMO are found to explain a large proportion of internal variability in Northern Hemisphere mean temperatures. Competition between a modest positive peak in the AMO and a substantially negative-trending PMO are seen to produce a slowdown or “false pause” in warming of the past decade.

Conclusion:

Our findings have strong implications for the attribution of recent climate changes. We find that internal multidecadal variability in Northern Hemisphere temperatures (the NMO), rather than having contributed to recent warming, likely offset anthropogenic warming over the past decade. … Given the pattern of past historical variation, this trend will likely reverse with internal variability instead, adding to anthropogenic warming in the coming decades.

The recent warming “hiatus” is subject to intense interest, with proposed causes including natural forcing and internal variability. Here we derive samples of all natural and internal variability from observations and a recent proxy reconstruction to investigate the likelihood that these two sources of variability could produce a hiatus or rapid warming in surface temperature. The likelihood is found to be consistent with that calculated previously for models and exhibits a similar spatial pattern, with an Interdecadal Pacific Oscillation-like structure, although with more signal in the Atlantic than in model patterns.

The number and length of events increases if natural forcing is also considered, particularly in the models. From the reconstruction it can be seen that large eruptions, such as Mount Tambora in 1815, or clusters of eruptions, may result in a hiatus of over 20 years, a finding supported by model results.

Every decade since the 1960s has been warmer than the one before, with 2000 to 2009 by far the warmest decade on record (see the figure). However, the role of human-induced climate change has been discounted by some, owing to a markedly reduced increase in global mean surface temperature (GMST) from 1998 through 2013, known as the hiatus (1–3). The upward trend has resumed in 2014, now the warmest year on record, with 2015 temperatures on course for another record-hot year. Although Earth’s climate is undoubtedly warming, weather-related and internal natural climate variability can temporarily overwhelm global warming in any given year or even decade, especially locally.

We investigate the relative magnitudes of the contributions of surface temperature trends from different latitude bands to the recent warming hiatus. We confirm from five different global datasets that the global-mean surface temperature trend in the period 1998–2012 is strongly influenced by a pronounced Eurasian winter cooling trend. To understand the drivers of this winter–cooling trend, we perform 3 twenty-member ensembles of simulations with different prescribed sea surface temperature and sea ice in the atmospheric model ECHAM6. Our experimental results suggest that the Arctic sea-ice loss does not drive systematic changes in the northern-hemisphere large-scale circulation in the past decades. The observed Eurasian winter cooling trend over 1998–2012 arises essentially from atmospheric internal variability and constitutes an extreme climate event. However, the observed reduction in Arctic sea ice enhances the variability of Eurasian winter climate and thus increases the probability of an extreme Eurasian winter cooling trend.

The reported “hiatus” in the warming of the global climate system during this century has been the subject of intense scientific and public debate, with implications ranging from scientific understanding of the global climate sensitivity to the rate in which greenhouse gas emissions would need to be curbed in order to meet the United Nations global warming target. A number of scientific hypotheses have been put forward to explain the hiatus, including both physical climate processes and data artifacts. However, despite the intense focus on the hiatus in both the scientific and public arenas, rigorous statistical assessment of the uniqueness of the recent temperature time-series within the context of the long-term record has been limited. We apply a rigorous, comprehensive statistical analysis of global temperature data that goes beyond simple linear models to account for temporal dependence and selection effects. We use this framework to test whether the recent period has demonstrated i) a hiatus in the trend in global temperatures, ii) a temperature trend that is statistically distinct from trends prior to the hiatus period, iii) a “stalling” of the global mean temperature, and iv) a change in the distribution of the year-to-year temperature increases.

We find compelling evidence that recent claims of a “hiatus” in global warming lack sound scientific basis. Our analysis reveals that there is no hiatus in the increase in the global mean temperature, no statistically significant difference in trends, no stalling of the global mean temperature, and no change in year-to-year temperature increases.

There has been much recent published research about a putative “pause” or “hiatus” in global warming. We show that there are frequent fluctuations in the rate of warming around a longer-term warming trend, and that there is no evidence that identifies the recent period as unique or particularly unusual. In confirmation, we show that the notion of a “pause” in warming is considered to be misleading in a blind expert test. Nonetheless, the most recent fluctuation about the longer-term trend has been regarded by many as an explanatory challenge that climate science must resolve. This departs from long-standing practice, insofar as scientists have long recognized that the climate fluctuates, that linear increases in CO2 do not produce linear trends in global warming, and that 15-year (or shorter) periods are not diagnostic of long-term trends.

We suggest that the repetition of the “warming has paused” message by contrarians was adopted by the scientific community in its problem-solving and answer-seeking role and has led to undue focus on, and mislabeling of, a recent fluctuation. We present an alternative framing that could have avoided inadvertently reinforcing a misleading claim.

Recent public debate and the scientific literature have frequently cited a “pause” or “hiatus” in global warming. Yet, multiple sources of evidence show that climate change continues unabated, raising questions about the status of the “hiatus”.

To examine whether the notion of a “hiatus” is justified by the available data, we first document that there are multiple definitions of the “hiatus” in the literature, with its presumed onset spanning a decade. For each of these definitions we compare the associated temperature trend against trends of equivalent length in the entire record of modern global warming. The analysis shows that the “hiatus” trends are encompassed within the overall distribution of observed trends. We next assess the magnitude and significance of all possible trends up to 25 years duration looking backwards from each year over the past 30 years. At every year during the past 30 years, the immediately preceding warming trend was always significant when 17 years (or more) were included in the calculation, alleged “hiatus” periods notwithstanding. If current definitions of the “pause” used in the literature are applied to the historical record, then the climate system “paused” for more than 1/3 of the period during which temperatures rose 0.6 K.

During the first decade of the twenty-first century, the Earth’s surface warmed more slowly than climate models simulated. This surface-warming hiatus is attributed by some studies to model errors in external forcing, while others point to heat rearrangements in the ocean caused by internal variability, the timing of which cannot be predicted by the models1. However, observational analyses disagree about which ocean region is responsible.

Here we show that the hiatus could also have been caused by internal variability in the top-of-atmosphere energy imbalance. Energy budgeting for the ocean surface layer over a 100-member historical ensemble reveals that hiatuses are caused by energy-flux deviations as small as 0.08 W m−2, which can originate at the top of the atmosphere, in the ocean, or both. Budgeting with existing observations cannot constrain the origin of the recent hiatus, because the uncertainty in observations dwarfs the small flux deviations that could cause a hiatus. The sensitivity of these flux deviations to the observational dataset and to energy budget choices helps explain why previous studies conflict, and suggests that the origin of the recent hiatus may never be identified.

“The Coupled Model Inter-comparison Project Phase 5 (CMIP5) contains a group of state-of-the-art climate models and represents the highest level of climate simulation thus far. However, these models significantly overestimated global mean surface temperature (GMST) during 2006–2014. Based on the ensemble empirical mode decomposition (EEMD) method, the long term change of the observed GMST time series of HadCRUT4 records during 1850–2014 was analyzed, then the simulated GMST by 33 CMIP5 climate models was assessed. The possible reason that climate models failed to project the recent global warming hiatus was revealed. Results show that during 1850–2014 the GMST on a centennial timescale rose with fluctuation, dominated by the secular trend and the multi-decadal variability (MDV). …

“This implies that the role of atmospheric CO2 in global warming may be overestimated, while the MDV which is an interior oscillation of the climate system may be underestimated, which should be related to insufficient understanding of key climatic internal dynamic processes. Our study puts forward an important criterion for the new generation of climate models: they should be able to simulate both the secular trend and the MDV of GMST.”

“The globally-averaged annual combined land and ocean surface temperature (GST) anomaly change features a slowdown in the rate of global warming in the mid-twentieth century and the beginning of the twenty-first century. Here, it is shown that the hiatus in the rate of global warming typically occurs when the internally generated cooling associated with the cool phase of the multi-decadal variability overcomes the secular warming from human-induced forcing.

“We provide compelling evidence that the global warming hiatus is a natural product of the interplays between a secular warming tendency due in a large part to the buildup of anthropogenic greenhouse gas concentrations, in particular CO2 concentration, and internally generated cooling by a cool phase of a quasi-60-year oscillatory variability that is closely associated with the Atlantic multi-decadal oscillation (AMO) and the Pacific decadal oscillation (PDO). We further illuminate that the AMO can be considered as a useful indicator and the PDO can be implicated as a harbinger of variations in global annual average surface temperature on multi-decadal timescales.

“Our results suggest that the recent observed hiatus in the rate of global warming will very likely extend for several more years due to the cooling phase of the quasi-60-year oscillatory variability superimposed on the secular warming trend.”

Increases in Asian aerosol emissions have been suggested as one possible reason for the hiatus in global temperature increase during the past 15 years. We study the effect of sulphur and black carbon (BC) emission changes between 1996 and 2010 on the global energy balance. We find that the increased Asian emissions have had very little regional or global effects, while the emission reductions in Europe and the U.S. have caused a positive radiative forcing. In our simulations, the global-mean aerosol direct radiative effect changes by 0.06 W/m2 during 1996 to 2010, while the effective radiative forcing (ERF) is 0.42 W/m2. The rather large ERF arises mainly from changes in cloudiness, especially in Europe. In Asia, the BC warming due to sunlight absorption has largely offset the cooling caused by sulphate aerosols. Asian BC concentrations have increased by a nearly constant fraction at all altitudes, and thus, they warm the atmosphere also in cloudy conditions.

Since the 1980s anthropogenic aerosols have been considerably reduced in Europe and the Mediterranean area. This decrease is often considered as the likely cause of the brightening effect observed over the same period. This phenomenon is however hardly reproduced by global and regional climate models.

Here we use an original approach based on reanalysis-driven coupled regional climate system modeling to show that aerosol changes explain 81 ± 16% of the brightening and 23 ± 5% of the surface warming simulated for the period 1980–2012 over Europe. The direct aerosol effect is found to dominate in the magnitude of the simulated brightening. The comparison between regional simulations and homogenized ground-based observations reveals that observed surface solar radiation and land and sea surface temperature spatiotemporal variations over the Euro-Mediterranean region are only reproduced when simulations include the realistic aerosol variations.

(6) Volcanoes

Observations suggest that the optical depth of the stratospheric aerosol layer between 20 and 30 km has increased 4–10% per year since 2000, which is significant for Earth’s climate. Contributions to this increase both from moderate volcanic eruptions and from enhanced coal burning in Asia have been suggested. Current observations are insufficient to attribute the contribution of the different sources. Here we use a global climate model coupled to an aerosol microphysical model to partition the contribution of each.

We employ model runs that include the increases in anthropogenic sulfur dioxide (SO2) over Asia and the moderate volcanic explosive injections of SO2 observed from 2000 to 2010. Comparison of the model results to observations reveals that moderate volcanic eruptions, rather than anthropogenic influences, are the primary source of the observed increases in stratospheric aerosol.

Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously. Possible explanations for the slow-down include internal climate variability, external cooling influences and observational errors. Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions to the muted surface warming.

Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere.

We show that climate model simulations without the effects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998. In two simulations with more realistic volcanic influences following the 1991 Pinatubo eruption, differences between simulated and observed tropospheric temperature trends over the period 1998 to 2012 are up to 15% smaller, with large uncertainties in the magnitude of the effect. To reduce these uncertainties, better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these eruption-specific properties in climate model simulations.

Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth (AOD) above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, AERONET and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at mid to high latitudes, and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be −0.19 ± 0.09 Wm−2. This translates into an estimated global cooling of 0.05 to 0.12 °C.

We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.

The relatively muted warming of the surface and lower troposphere since 1998 has attracted considerable attention. One contributory factor to this “warming hiatus” is an increase in volcanically-induced cooling over the early 21st century. Here, we identify the signals of late 20th and early 21st century volcanic activity in multiple observed climate variables. Volcanic signals are statistically discernible in spatial averages of tropical and near-global SST, tropospheric temperature, net clear-sky short-wave radiation, and atmospheric water vapor. Signals of late 20th and early 21st century volcanic eruptions are also detectable in near-global averages of rainfall. In tropical-average rainfall, however, only a Pinatubo-caused drying signal is identifiable. Successful volcanic signal detection is critically dependent on removal of variability induced by the El Niño/Southern Oscillation (ENSO).

(7) Changes in the oceans and ocean-air cycles

The eminent Roger Pielke Sr (see Wikipedia) has long said that the focus on the surface air temperature was inappropriate, and that…

For this he was smeared, and called a denier by activists. Such as those at Skeptical Science (Dana Nuccitelli’s launch pad, which should be called “skeptical of science”). See this page calling him a “climate misinformer” (note that all of his quotes shown there now appear correct). See this note for a more detail, and references to his work.

Now that the surface temperature has paused, climate scientists have realized that he was correct. And of course activists give no retraction or apologies for their smears. Now back to the scientists …

Despite a sustained production of anthropogenic greenhouse gases, the Earth’s mean near-surface temperature paused its rise during the 2000–2010 period. To explain such a pause, an increase in ocean heat uptake below the superficial ocean layer has been proposed to overcompensate for the Earth’s heat storage. Contributions have also been suggested from the deep prolonged solar minimum, the stratospheric water vapour, the stratospheric and tropospheric aerosols. However, a robust attribution of this warming slowdown has not been achievable up to now.

Here we show successful retrospective predictions of this warming slowdown up to 5 years ahead, the analysis of which allows us to attribute the onset of this slowdown to an increase in ocean heat uptake. Sensitivity experiments accounting only for the external radiative forcings do not reproduce the slowdown. The top-of-atmosphere net energy input remained in the [0.5–1] W m−2 interval during the past decade, which is successfully captured by our predictions. Most of this excess energy was absorbed in the top 700 m of the ocean at the onset of the warming pause, 65% of it in the tropical Pacific and Atlantic oceans. Our results hence point at the key role of the ocean heat uptake in the recent warming slowdown.

The ability to predict retrospectively this slowdown not only strengthens our confidence in the robustness of our climate models, but also enhances the socio-economic relevance of operational decadal climate predictions.

Global warming first became evident beyond the bounds of natural variability in the 1970s, but increases in global mean surface temperatures have stalled in the 2000s. Increases in atmospheric greenhouse gases, notably carbon dioxide, create an energy imbalance at the top-of-atmosphere (TOA) even as the planet warms to adjust to this imbalance, which is estimated to be 0.5–1 W m−2 over the 2000s.

Annual global fluctuations in TOA energy of up to 0.2 W m−2 occur from natural variations in clouds, aerosols, and changes in the Sun. At times of major volcanic eruptions the effects can be much larger. Yet global mean surface temperatures fluctuate much more than these can account for. An energy imbalance is manifested not just as surface atmospheric or ground warming but also as melting sea and land ice, and heating of the oceans.

More than 90% of the heat goes into the oceans and, with melting land ice, causes sea level to rise. For the past decade, more than 30% of the heat has apparently penetrated below 700 m depth that is traceable to changes in surface winds mainly over the Pacific in association with a switch to a negative phase of the Pacific Decadal Oscillation (PDO) in 1999. Surface warming was much more in evidence during the 1976–1998 positive phase of the PDO, suggesting that natural decadal variability modulates the rate of change of global surface temperatures while sea-level rise is more relentless.

Global warming has not stopped; it is merely manifested in different ways.

The biggest mystery in climate science today may have begun, unbeknownst to anybody at the time, with a subtle weakening of the tropical trade winds blowing across the Pacific Ocean in late 1997. These winds normally push sun-baked water towards Indonesia. When they slackened, the warm water sloshed back towards South America, resulting in a spectacular example of a phenomenon known as El Niño. Average global temperatures hit a record high in 1998 — and then the warming stalled.

… Now, as the global-warming hiatus enters its 16th year, scientists are at last making headway in the case of the missing heat. Some have pointed to the Sun, volcanoes and even pollution from China as potential culprits, but recent studies suggest that the oceans are key to explaining the anomaly. The latest suspect is the El Niño of 1997–98, which pumped prodigious quantities of heat out of the oceans and into the atmosphere — perhaps enough to tip the equatorial Pacific into a prolonged cold state that has suppressed global temperatures ever since.

This is a remarkable article, especially in one of the premier science journals. The role of ocean-atmosphere cycles (e.g., PDO, AMO, ENSO) in warming has been a major theme of climate skeptics — of which Bob Tisdale is the best-known. Activists have smeared and mocked them, so recognition of the validity of their insights is a stunning vindication, even if uncredited in this article. This is a common pattern in the history of science, professionals following path-breaking work by amateurs.

The El Niño – Southern Oscillation is known to influence surface temperatures worldwide. El Niño conditions are thought to lead to anomalously warm global average surface temperature, absent other forcings. Recent research has identified distinct possible types of El Niño events based on the location of peak sea surface temperature anomalies.

Here we analyze the relationship between the type of El Niño event and the global surface average temperature anomaly, using three historical temperature data sets. Separating El Niño events into types reveals that the global average surface temperatures are anomalously warm during and after traditional eastern Pacific El Niño events, but not central Pacific or mixed events. Historical analysis indicated that slowdowns in the rate of global surface warming since the late 1800s may be related to decadal variability in the frequency of different types of El Niño events.

(e) For a review of the literature on changes in the oceans’ heat content see this article by Judith Curry (Prof Climate Science, GA Institute of Tech). She points to this paper in the Journal of Climate (March 2014) describing the large uncertainties in measurements of the ocean heat content.

The globally integrated heat content changes involve small differences of the much larger regional changes. As existing estimates of the anthropogenic forcing are now about 0.5W/m2, the equivalent global ocean average temperature changes over 20 years are mostly slight compared to the shorter term temporal variations from numerous physical sources. Detailed attention must be paid to what might otherwise appear to be small errors in data calibration, and space-time sampling and model biases. Direct determination of changes in oceanic heat content over the last 20 years are not in conflict with estimates of the radiative forcing, but the uncertainties remain too large to rationalize e.g., the apparent “pause” in warming.

A vacillating global heat sink at intermediate ocean depths is associated with different climate regimes of surface warming under anthropogenic forcing: The latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake. In addition to the shallow La Niña–like patterns in the Pacific that were the previous focus, we found that the slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic. Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years.

The first decade of the twenty-first century was characterised by a hiatus in global surface warming. Using ocean model hindcasts and reanalyses we show that heat uptake between the 1990s and 2000s increased by 0.7 ± 0.3Wm−2. Approximately 30% of the increase is associated with colder sea surface temperatures in the eastern Pacific. Other basins contribute via reduced heat loss to the atmosphere, in particular the Southern and subtropical Indian Oceans (30%), and the subpolar North Atlantic (40%). A different mechanism is important at longer timescales (1960s-present) over which the Southern Annular Mode trended upwards. In this period, increased ocean heat uptake has largely arisen from reduced heat loss associated with reduced winds over the Agulhas Return Current and southward displacement of Southern Ocean westerlies.

Global mean surface warming has stalled since the end of the twentieth century, but the net radiation imbalance at the top of the atmosphere continues to suggest an increasingly warming planet. This apparent contradiction has been reconciled by an anomalous heat flux into the ocean induced by a shift towards a La Niña-like state with cold sea surface temperatures in the eastern tropical Pacific over the past decade or so. A significant portion of the heat missing from the atmosphere is therefore expected to be stored in the Pacific Ocean. However, in situ hydrographic records indicate that Pacific Ocean heat content has been decreasing.

Here, we analyse observations along with simulations from a global ocean–sea ice model to track the pathway of heat. We find that the enhanced heat uptake by the Pacific Ocean has been compensated by an increased heat transport from the Pacific Ocean to the Indian Ocean, carried by the Indonesian throughflow. As a result, Indian Ocean heat content has increased abruptly, which accounts for more than 70% of the global ocean heat gain in the upper 700 m during the past decade. We conclude that the Indian Ocean has become increasingly important in modulating global climate variability.

A new study, by scientists from the University of Southampton and National Oceanography Centre (NOC), implies that the global climate is on the verge of broad-scale change that could last for a number of decades. …

The strength of ocean currents has been measured by a network of sensors, called the RAPID array, which have been collecting data on the flow rate of the Atlantic meridonal overturning circulation (AMOC) for a decade. Dr David Smeed, from the NOC and lead scientist of the RAPID project, adds: “The observations of AMOC from the RAPID array, over the past ten years, show that it is declining. As a result, we expect the AMO is moving to a negative phase, which will result in cooler surface waters. This is consistent with observations of temperature in the North Atlantic.”

Abstract:

Decadal variability is a notable feature of the Atlantic Ocean and the climate of the regions it influences. Prominently, this is manifested in the Atlantic Multidecadal Oscillation (AMO) in sea surface temperatures. Positive (negative) phases of the AMO coincide with warmer (colder) North Atlantic sea surface temperatures. The AMO is linked with decadal climate fluctuations, such as Indian and Sahel rainfall, European summer precipitation, Atlantic hurricanes and variations in global temperatures.

It is widely believed that ocean circulation drives the phase changes of the AMO by controlling ocean heat content. However, there are no direct observations of ocean circulation of sufficient length to support this, leading to questions about whether the AMO is controlled from another source.

Here we provide observational evidence of the widely hypothesized link between ocean circulation and the AMO. We take a new approach, using sea level along the east coast of the United States to estimate ocean circulation on decadal timescales. We show that ocean circulation responds to the first mode of Atlantic atmospheric forcing, the North Atlantic Oscillation, through circulation changes between the subtropical and subpolar gyres — the intergyre region. These circulation changes affect the decadal evolution of North Atlantic heat content and, consequently, the phases of the AMO.

The Atlantic overturning circulation is declining and the AMO is moving to a negative phase. This may offer a brief respite from the persistent rise of global temperatures, but in the coupled system we describe, there are compensating effects. In this case, the negative AMO is associated with a continued acceleration of sea-level rise along the northeast coast of the United States.

Recent modeling studies have proposed different scenarios to explain the slowdown in surface temperature in the most recent decade. Some of these studies seem to support the idea of internal variability and/or rearrangement of heat between the surface and the ocean interior. Others suggest that radiative forcing might also play a role. Our examination of observational data over the past two decades shows some significant differences compared to model results from reanalyses, and provides the most definitive explanation of how the heat was redistributed. We find that cooling in the top 100-meter layer of the Pacific Ocean was mainly compensated by warming in the 100- to 300-meter layer of the Indian and Pacific Oceans in the past decade since 2003.

Global mean surface temperatures (GMST) exhibited a smaller rate of warming during 1998–2013, compared to the warming in the latter half of the 20th Century. Although, not a “true” hiatus in the strict definition of the word, this has been termed the “global warming hiatus” by IPCC (2013). There have been other periods that have also been defined as the “hiatus” depending on the analysis. There are a number of uncertainties and knowledge gaps regarding the “hiatus.”

This report reviews these issues and also posits insights from a collective set of diverse information that helps us understand what we do and do not know. One salient insight is that the GMST phenomenon is a surface characteristic that does not represent a slowdown in warming of the climate system but rather is an energy redistribution within the oceans. Improved understanding of the ocean distribution and redistribution of heat will help better monitor Earth’s energy budget and its consequences.

A review of recent scientific publications on the “hiatus” shows the difficulty and complexities in pinpointing the oceanic sink of the “missing heat” from the atmosphere and the upper layer of the oceans, which defines the “hiatus.” Advances in “hiatus” research and outlooks (recommendations) are given in this report.

“The recent slowdown in global warming challenged our understanding of climate dynamics and anthropogenic forcing. An early study gave insight to the mechanisms behind the warming slowdown and highlighted the ocean’s role in regulating global temperature.”

This study investigates spatio-temporal features of multidecadal climate variability, using observations and climate model simulation. Aside from a long-term warming trend, observational SST and atmospheric circulation records are dominated by a ~65yr variability component. Though its center of action is over the North Atlantic, but it manifests also over the Pacific and Indian Oceans, suggesting a tropical inter-basin teleconnection maintained through an atmospheric bridge.

Our analysis shows that simulated internal climate variability in a coupled climate model (CSIRO-Mk3.6.0) reproduces the main spatio-temporal features of the observed component. Model-based multidecadal variability comprises a coupled ocean-atmosphere teleconnection, established through a zonally oriented atmospheric overturning circulation between the tropical North Atlantic and eastern tropical Pacific. During the warm SST phase in the North Atlantic, increasing SSTs over the tropical North Atlantic strengthen locally ascending air motion and intensify subsidence and low-level divergence in the eastern tropical Pacific. This corresponds with a strengthening of trade winds and cooling in the tropical central Pacific.

The model’s derived component substantially shapes its global climate variability and is tightly linked to multidecadal variability of the Atlantic Meridional Overturning Circulation (AMOC). This suggests potential predictive utility and underscores the importance of correctly representing North Atlantic variability in simulations of global and regional climate.

If the observations-based component of variability originates from internal climate processes, as found in the model, the recently observed (1970s-2000s) North Atlantic warming and eastern tropical Pacific cooling might presage an ongoing transition to a cold North Atlantic phase with possible implications for near-term global temperature evolution.

Slower rates of increase in global mean surface temperature (GMST) after 2000, dubbed “global warming hiatus,” recently gave way to a rapid temperature rise. This rise coincided with persistent warm conditions in the equatorial Pacific between March 2014 and May 2016, which peaked as the 2015 extreme El Niño. Here we show that the El Niño–Southern Oscillation (ENSO) tightly controls interannual variations in atmospheric heating rate in the tropics (r > 0.9), allowing us to construct a simple, physically based model of GMST variations that incorporates greenhouse gas emissions, ENSO forcing, and stratospheric sulfate aerosols produced by volcanoes.

The model closely reproduces GMST changes since 1880, including the global warming hiatus and the subsequent temperature rise. Our results confirm that weak El Niño activity, rather than volcanic eruptions, was the cause of the hiatus, while the rapid temperature rise is due to atmospheric heat release during 2014–2016 El Niño conditions concurrent with the continuing global warming trend.

“A surface cooling pattern in the equatorial Pacific associated with a negative phase of the Interdecadal Pacific Oscillation is the leading hypothesis to explain the smaller rate of global warming during 1998-2014, with these cooler than normal conditions thought to have accelerated the oceanic heat uptake. Here, using a 30-member ensemble simulation of a global Earth system model, we show that in 10% of all simulated decades with a global cooling trend, the eastern equatorial Pacific actually warms.

“Our finding challenges the view of the equatorial Pacific being the sole pacemaker for generating internal stochastic variability-driven global warming hiatus decades and suggests that past and future surface temperature patterns during hiatus decades may be distinct. In addition, the global ocean heat uptake tends to slow down during hiatus decades implying a fundamentally different global climate feedback factor on decadal timescales than on centennial timescales and calling for caution inferring climate sensitivity from decadal-scale variability.”

A hypothesized low-frequency climate signal propagating across the Northern Hemisphere through a network of synchronized climate indices was identified in previous analyses of instrumental and proxy data. The tempo of signal propagation is rationalized in terms of the multidecadal component of Atlantic Ocean variability — the Atlantic Multidecadal Oscillation. Through multivariate statistical analysis of an expanded database, we further investigate this hypothesized signal to elucidate propagation dynamics.

The Eurasian Arctic Shelf-Sea Region, where sea ice is uniquely exposed to open ocean in the Northern Hemisphere, emerges as a strong contender for generating and sustaining propagation of the hemispheric signal. Ocean-ice-atmosphere coupling spawns a sequence of positive and negative feedbacks that convey persistence and quasi-oscillatory features to the signal. Further stabilizing the system are anomalies of co-varying Pacific-centered atmospheric circulations.

Indirectly related to dynamics in the Eurasian Arctic, these anomalies appear to negatively feed back onto the Atlantic‘s freshwater balance. Earth’s rotational rate and other proxies encode traces of this signal as it makes its way across the Northern Hemisphere.

For the past 15+ years, there has been no increase in global average surface temperature, which has been referred to as a ‘hiatus’ in global warming. By contrast, estimates of expected warming in the first several decades of 21st century made by the IPCC AR4 were 0.2C/decade. This talk summarizes the recent CMIP5 climate model simulation results and comparisons with observational data.

The most recent climate model simulations used in the AR5 indicate that the warming stagnation since 1998 is no longer consistent with model projections even at the 2% confidence level. Potential causes for the model-observation discrepancies are discussed. A particular focus of the talk is the role of multi-decadal natural internal variability on the climate variability of the 20th and early 21st centuries. The “stadium wave” climate signal is described, which propagates across the Northern Hemisphere through a network of ocean, ice, and atmospheric circulation regimes that self-organize into a collective tempo. The stadium wave hypothesis provides a plausible explanation for the hiatus in warming and helps explain why climate models did not predict this hiatus. Further, the new hypothesis suggests how long the hiatus might last. Implications of the hiatus are discussed in context of climate model sensitivity to CO2 forcing and attribution of the warming that was observed in the last quarter of the 20th century.

The warming of the climate system is unequivocal as evidenced by an increase in global temperatures by 0.8 °C over the past century. However, the attribution of the observed warming to human activities remains less clear, particularly because of the apparent slow-down in warming since the late 1990s.

Here we analyse radiative forcing and temperature time series with state-of-the-art statistical methods to address this question without climate model simulations. We show that long-term trends in total radiative forcing and temperatures have largely been determined by atmospheric greenhouse gas concentrations, and modulated by other radiative factors. We identify a pronounced increase in the growth rates of both temperatures and radiative forcing around 1960, which marks the onset of sustained global warming.

Our analyses also reveal a contribution of human interventions to two periods when global warming slowed down. Our statistical analysis suggests that the reduction in the emissions of ozone-depleting substances under the Montreal Protocol, as well as a reduction in methane emissions, contributed to the lower rate of warming since the 1990s. Furthermore, we identify a contribution from the two world wars and the Great Depression to the documented cooling in the mid-twentieth century, through lower carbon dioxide emissions.

We conclude that reductions in greenhouse gas emissions are effective in slowing the rate of warming in the short term.

(10) It’s the reduced solar activity

Many scientists have pointed to changes in solar activity as drivers of decadal or century-long climate cycles (see research listed in section #7 here). Climate activists have denounced as “deniers” anyone daring to mention this research (see the comments to the posts about solar cycles). Now that’s gone down the memory hole, as solar activity becomes an explanation for the pause (we’ve always been at war with EastAsia). Back to the science…

The question whether human activities seriously affect climate is asked with increasing voice these days. Quite understandable since the climate appears to be out of control with the significant global temperature increases already seen during the last 3 decades and with still heavier temperature increases to come in the future according to prognoses, among others, in the recent comprehensive IPCC reports.

However, the most recent climate data, show global temperature development levelling off or even turning negative since 2001 in contrast to the anticipated course related to the steady increases in the concentration in the atmosphere of green-house gasses, primarily carbon dioxide and methane.

The purpose of this communication is to demonstrate that the reduced rate in the global temperature rise complies with expectations related to the decaying level of solar activity according to the relation published in an earlier analysis. Without the reduction in the solar activity-related contributions the global temperatures would have increased steadily from 1980 to present.

(11) Some of the global temperature datasets underestimates warming

Doubting the accuracy of global temperature measurements has been considered heresy by climate activists (denier, denier they’d cry). But desperate for salvation they greeted this paper with euphoric celebration, although much of the initial response by climate scientists was critical (examples here). Plus, of course, there are three other major temperature datasets.

In complete global coverage is a potential source of bias in global temperature reconstructions if the unsampled regions are not uniformly distributed over the planet’s surface. The widely used HadCRUT4 dataset covers on average about 84% of the globe over recent decades, with the unsampled regions being concentrated at the poles and over Africa.

Three existing reconstructions with near-global coverage are examined, each suggesting that HadCRUT4 is subject to bias due to its treatment of unobserved regions. Two alternative approaches for reconstructing global temperatures are explored, one based on an optimal interpolation algorithm and the other a hybrid method incorporating additional information from the satellite temperature record. The methods are validated on the basis of their skill at reconstructing omitted sets of observations. Both methods provide superior results than excluding the unsampled regions, with the hybrid method showing particular skill around the regions where no observations are available.

Temperature trends are compared for the hybrid global temperature reconstruction and the raw HadCRUT4 data. The widely quoted trend since 1997 in the hybrid global reconstruction is two and a half times greater than the corresponding trend in the coverage-biased HadCRUT4 data. Coverage bias causes a cool bias in recent temperatures relative to the late 1990s which increases from around 1998 to the present. Trends starting in 1997 or 1998 are particularly biased with respect to the global trend. The issue is exacerbated by the strong El Ni ̃no event of 1997-1998, which also tends to suppress trends starting during those years.

Over the last 15 years, global mean surface temperatures exhibit only weak trends. Recent studies have attempted to attribute this so called temperature hiatus to several causes, amongst them incomplete sampling of the rapidly warming Arctic region. We here examine zonal mean temperature trends in satellite-based tropospheric data sets (based on data from (Advanced) Microwave Sounding Unit and Global Navigation Satellite System Radio Occultation instruments) and in global surface temperatures (HadCRUT4). Omission of successively larger polar regions from the global mean temperature calculations, in both tropospheric and surface data sets, shows that data gaps at high latitudes cannot explain the observed differences between the hiatus and the prehiatus period.

Instead, the dominating causes of the global temperature hiatus are found at low latitudes. The combined use of several independent data sets, representing completely different measurement techniques and sampling characteristics, strengthens the conclusions.

Near-surface and lower-tropospheric warming of the Arctic over the past 35 years is examined for several datasets. The new estimate for the near surface reported by Cowtan and Way in 2014 agrees reasonably well with the ERA-Interim reanalysis for this region. Both provide global averages with a little more warming over recent years than indicated by the widely used HadCRUT4 dataset, which has sparse coverage of the high Arctic. ERA-Interim is more sensitive than the Cowtan and Way estimate to the state of the underlying Arctic Ocean.

Observational coverage of the Arctic varies considerably over the period. Surface air-temperature data of identified types are generally fitted well by ERA-Interim, especially data from ice stations, which appear of excellent quality. ERA-Interim nevertheless has a warm wintertime bias over sea-ice. Mean fits vary in magnitude as coverage varies, but their overall changes are much smaller than analysed temperature changes. This is also largely the case for fits to data for the free troposphere. Much of the information on trends and low-frequency variability provided by ERA-Interim comes from its background forecast, which carries forward information assimilated from a rich variety of earlier observations, rather than from its analysis of surface air-temperature observations.

ERA-Interim agrees quite well with the new JRA-55 reanalysis, and with the MERRA reanalysis until recent years when MERRA exhibits weaker surface warming. Temperatures vary coherently between the surface and middle troposphere, with largest amplitude at the surface except in summer, when air temperatures are constrained by sea-ice and open-sea temperatures that differ little from 0°C. Much of the recent near-surface warming of the Arctic is associated with reduced cold-season sea-ice cover, with low temperatures over ice replaced by much higher ones over open sea. This occurs primarily in a relatively well-observed region around the northernmost islands of Europe and western Asia.

The warming hiatus shows a strong seasonal and geographical asymmetry, with cooling in the Northern Hemisphere winter, especially over land, and warming elsewhere and in the other seasons. We show that the characteristics of the Northern Hemisphere winter cooling in 1998–2012 can mostly be explained by missing observations and by internal variability in the atmospheric circulation of the Northern Hemisphere extratropics. Estimates of the annual and seasonal temperature trends in 1998–2012 obtained by considering the concurrent effects of unforced natural variability and of coverage bias are much closer to the corresponding long-term trends.

Reanalyses suggest that the coverage bias was exceptionally pronounced during recent years and that an area of strong warming was missed due to the incomplete observational coverage. Coupled Model Intercomparison Project Phase 5 climate models indicate that trends in atmospheric circulation during the hiatus period did not occur as a response to anthropogenic forcing.

Much study has been devoted to the possible causes of an apparent decrease in the upward trend of global surface temperatures since 1998, a phenomenon that has been dubbed the global warming “hiatus.” Here, we present an updated global surface temperature analysis that reveals that global trends are higher than those reported by the Intergovernmental Panel on Climate Change, especially in recent decades, and that the central estimate for the rate of warming during the first 15 years of the 21st century is at least as great as the last half of the 20th century. These results do not support the notion of a “slowdown” in the increase of global surface temperature.

Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing.

Here we show that a pronounced strengthening in Pacific trade winds over the past two decades — unprecedented in observations/reanalysis data and not captured by climate models — is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline.

At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions.

The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.

Their work is part of the ongoing shift of climate scientists’ expectations away from the forecasts of extreme warming. Excerpt:

… model is equilibrated for more than 3,000 years with atmospheric CO2 fixed at pre-industrial and then integrated during 1780 – 2030 following historical CO2 forcing (1780 – 2000) and then the CMIP3 A2 emissions scenario 46 from the year 2000 onwards. The model’s overall climate sensitivity is at the low end range of CMIP3 models (reaching 2.1 C warming by 2090 – 2099 relative to 1980 – 1999); …

Surface global warming has stalled since around 2000 despite increasing atmospheric CO2. A study finds that recent strengthening of Pacific trade winds has enhanced heat transport from the surface to ocean depths, explaining most of the slowed surface warming.

Reasons for the apparent pause in the rise of global-mean surface air temperature (SAT) after the turn of the century has been a mystery, undermining confidence in climate projections. Recent climate model simulations indicate this warming hiatus originated from eastern equatorial Pacific cooling associated with strengthening of trade winds. Using a climate model that overrides tropical wind stress anomalies with observations for 1958–2012, we show that decadal-mean anomalies of global SAT referenced to the period 1961–1990 are changed by 0.11, 0.13 and −0.11 °C in the 1980s, 1990s and 2000s, respectively, without variation in human-induced radiative forcing. They account for about 47%, 38% and 27% of the respective temperature change.

The dominant wind stress variability consistent with this warming/cooling represents the deceleration/acceleration of the Pacific trade winds, which can be robustly reproduced by atmospheric model simulations forced by observed sea surface temperature excluding anthropogenic warming components. Results indicate that inherent decadal climate variability contributes considerably to the observed global-mean SAT time series, but that its influence on decadal-mean SAT has gradually decreased relative to the rising anthropogenic warming signal.

Feedbacks of clouds on climate change strongly influence the magnitude of global warming. Cloud feedbacks, in turn, depend on the spatial patterns of surface warming which vary on decadal timescales. Therefore, the magnitude of the decadal cloud feedback could deviate from the long-term cloud feedback4. Here we present climate model simulations to show that the global mean cloud feedback in response to decadal temperature fluctuations varies dramatically due to time variations in the spatial pattern of sea surface temperature. We find that cloud anomalies associated with these patterns significantly modify the Earth’s energy budget. Specifically, the decadal cloud feedback between the 1980s and 2000s is substantially more negative than the long-term cloud feedback. This is a result of cooling in tropical regions where air descends, relative to warming in tropical ascent regions, which strengthens low-level atmospheric stability. Under these conditions, low-level cloud cover and its reflection of solar radiation increase, despite an increase in global mean surface temperature.

These results suggest that sea surface temperature pattern-induced low cloud anomalies could have contributed to the period of reduced warming between 1998 and 2013, and offer a physical explanation of why climate sensitivities estimated from recently observed trends are probably biased low.

(14) Multiple causes, unknown causes, or other aspects

(a) “Overestimated global warming over the past 20 years“, John C. Fyfe, Nathan P. Gillett and Francis W. Zwiers, Nature Climate Change, September 2013 — Open copy here. “Recent observed global warming is significantly less than that simulated by climate models. This difference might be explained by some combination of errors in external forcing, model response and internal climate variability.”

The latest generation of climate model simulations are used to investigate the occurrence of hiatus periods in global surface air temperature in the past and under two future warming scenarios. Hiatus periods are identified in three categories:

those due to volcanic eruptions,

those associated with negative phases of the Interdecadal Pacific Oscillation (IPO), and

those affected by anthropogenically released aerosols in the mid-twentieth century.

The likelihood of future hiatus periods is found to be sensitive to the rate of change of anthropogenic forcing. Under high rates of greenhouse gas emissions there is little chance of a hiatus decade occurring beyond 2030, even in the event of a large volcanic eruption. We further demonstrate that most nonvolcanic hiatuses across Coupled Model Intercomparison Project 5 (CMIP5) models are associated with enhanced cooling in the equatorial Pacific linked to the transition to a negative IPO phase.

Global mean surface warming over the past 15 years or so has been less than in earlier decades and than simulated by most climate models1. Natural variability, a reduced radiative forcing5, a smaller warming response to atmospheric carbon dioxide concentrations and coverage bias in the observations10 have been identified as potential causes. However, the explanations of the so-called ‘warming hiatus’ remain fragmented and the implications for long-term temperature projections are unclear. Here we estimate the contribution of internal variability associated with the El Niño/Southern Oscillation (ENSO) using segments of unforced climate model control simulations that match the observed climate variability. We find that ENSO variability analogous to that between 1997 or 1998 and 2012 leads to a cooling trend of about −0.06 °C.

In addition, updated solar and stratospheric aerosol forcings from observations explain a cooling trend of similar magnitude (−0.07 °C). Accounting for these adjusted trends we show that a climate model of reduced complexity with a transient climate response of about 1.8 °C is consistent with the temperature record of the past 15 years, as is the ensemble mean of the models in the Coupled Model Intercomparison Project Phase 5 (CMIP5). We conclude that there is little evidence for a systematic overestimation of the temperature response to increasing atmospheric CO2 concentrations in the CMIP5 ensemble.

Global surface temperature has been increasing since the beginning of the 20th century but with a highly variable warming rate, and the alternation of rapid warming periods with ‘hiatus’ decades is a constant throughout the series. The superimposition of a secular warming trend with natural multidecadal variability is the most accepted explanation for such a pattern.

Since the start of the 21st century, the surface global mean temperature has not risen at the same rate as the top-of-atmosphere radiative energy input or greenhouse gas emissions, provoking scientific and social interest in determining the causes of this apparent discrepancy. Multidecadal natural variability is the most commonly proposed cause for the present hiatus period.

Here, we analyze the HadCRUT4 surface temperature database with spectral techniques to separate a multidecadal oscillation (MDV) from a secular trend (ST). Both signals combined account for nearly 88% of the total variability of the temperature series showing the main acceleration/deceleration periods already described elsewhere. Three stalling periods with very little warming could be found within the series, from 1878 to 1907, from 1945 to 1969 and from 2001 to the end of the series, all of them coincided with a cooling phase of the MDV. Henceforth, MDV seems to be the main cause of the different hiatus periods shown by the global surface temperature records. However, and contrary to the two previous events, during the current hiatus period, the ST shows a strong fluctuation on the warming rate, with a large acceleration (0.0085°C year−1 to 0.017°C year−1) during 1992–2001 and a sharp deceleration (0.017°C year−1 to 0.003°C year−1) from 2002 onwards.

This is the first time in the observational record that the ST shows such variability, so determining the causes and consequences of this change of behavior needs to be addressed by the scientific community.

Despite a steady increase in atmospheric greenhouse gases (GHGs), global-mean surface temperature (T) has shown no discernible warming since about 2000, in sharp contrast to model simulations, which on average project strong warming. The recent slowdown in observed surface warming has been attributed to decadal cooling in the tropical Pacific, intensifying trade winds, changes in El Niño activity, increasing volcanic activity, and decreasing solar irradiance. Earlier periods of arrested warming have been observed but received much less attention than the recent period, and their causes are poorly understood.

“Here we analyse observed and model-simulated global T fields to quantify the contributions of internal climate variability (ICV) to decadal changes in global-mean T since 1920. We show that the Interdecadal Pacific Oscillation (IPO) has been associated with large T anomalies over both ocean and land. Combined with another leading mode of ICV, the IPO explains most of the difference between observed and model-simulated rates of decadal change in global-mean T since 1920, and particularly over the so-called ‘hiatus’ period since about 2000. We conclude that ICV, mainly through the IPO, was largely responsible for the recent slowdown, as well as for earlier slowdowns and accelerations in global-mean T since 1920, with preferred spatial patterns different from those associated with GHG-induced warming or aerosol-induced cooling. Recent history suggests that the IPO could reverse course and lead to accelerated global warming in the coming decades.”

(f) “Making sense of the early-2000s warming slowdown” by John C. Fife and a prestigious group of authors, Natural Climate Change, March 2016. Gated; open copy here. Excerpt, from the conclusions:

Our results support previous findings of a reduced rate of surface warming over the 2001–2014 period — a period in which anthropogenic forcing increased at a relatively constant rate. … Newly identified observational errors do not, however, negate the existence of a real reduction in the surface warming rate in the early twenty-first century relative to the 1970s–1990s. This reduction arises through the combined effects of internal decadal variability, volcanic and solar activity, and decadal changes in anthropogenic aerosol forcing.

The rate at which the global average surface temperature is increasing has slowed down since the end of the last century. This study investigates whether this warming hiatus results from a change in the well-known greenhouse effect. Using long-term, reliable, and consistent observational data from the Earth’s surface and the top of the atmosphere (TOA), two monthly gridded atmospheric and surface greenhouse effect parameters (Ga and Gs) are estimated to represent the radiative warming effects of the atmosphere and the surface in the infrared range from 1979 to 2014.

The atmospheric and surface greenhouse effect over the tropical monsoon-prone regions is found to contribute substantially to the global total. Furthermore, the downward tendency of cloud activity leads to a greenhouse effect hiatus after the early 1990 s, prior to the warming pause.

Additionally, this pause in the greenhouse effect is mostly caused by the high number of La Niña events between 1991 and 2014. A strong La Niña indicates suppressed convection in the tropical central Pacific that reduces atmospheric water vapor content and cloud volume. This significantly weakened regional greenhouse effect offsets the enhanced warming influence in other places and decelerates the rising global greenhouse effect.

This work suggests that the greenhouse effect hiatus can be served as an additional factor to cause the recent global warming slowdown.

“This study reviews the warming hiatus and identifies the key factors determining the spatial pattern of the warming hiatus. The warming hiatus over land is more notable in the US, Canada and the mid-latitudes of Eurasia and is especially evident in winter. This warming hiatus may be closely related to the negative phases of the NAO and AO and significant propagation of cold air into mid-latitudes by the northerly wind anomaly originating from the Arctic. The three dimensional structure of circulation associated with AO is first investigated here to clarify the dynamic processes leading to recent winter cooling in mid-latitude of Northern Hemisphere land.”

“Global surface warming was slower than expected in the first decade of the twenty-first century. Research attributes similar events to ocean or atmosphere fluctuations, but the subtle origins of these events may elude observational detection.”

“Between about 1998 and 2012, a time that coincided with political negotiations for preventing climate change, the surface of Earth seemed hardly to warm. This phenomenon, often termed the ‘global warming hiatus’, caused doubt in the public mind about how well anthropogenic climate change and natural variability are understood. Here we show that apparently contradictory conclusions stem from different definitions of ‘hiatus’ and from different datasets. A combination of changes in forcing, uptake of heat by the oceans, natural variability and incomplete observational coverage reconciles models and data. Combined with stronger recent warming trends in newer datasets, we are now more confident than ever that human influence is dominant in long-term warming.”

Interactions between externally forced and internally generated climate variations on decadal timescales is a major determinant of changing climate risk. Severe testing is applied to observed global and regional surface and satellite temperatures and modelled surface temperatures to determine whether these interactions are independent, as in the traditional signal-to-noise model, or whether they interact, resulting in step-like warming.

The multistep bivariate test is used to detect step changes in temperature data. The resulting data are then subject to six tests designed to distinguish between the two statistical hypotheses, hstep and htrend.

Test 1: since the mid-20th century, most observed warming has taken place in four events: in 1979/80 and 1997/98 at the global scale, 1988/89 in the Northern Hemisphere and 1968–70 in the Southern Hemisphere. Temperature is more step-like than trend-like on a regional basis. Satellite temperature is more step-like than surface temperature. Warming from internal trends is less than 40 % of the total for four of five global records tested (1880–2013/14).

Test 2: correlations between step-change frequency in observations and models (1880–2005) are 0.32 (CMIP3) and 0.34 (CMIP5). For the period 1950–2005, grouping selected events (1963/64, 1968–70, 1976/77, 1979/80, 1987/88 and 1996–98), the correlation increases to 0.78.

Test 4: in three regions tested, the change between stationary and non-stationary temperatures is step-like and attributable to external forcing.

Test 5: step-like changes are also present in tide gauge observations, rainfall, ocean heat content and related variables. Test 6: across a selection of tests, a simple stepladder model better represents the internal structures of warming than a simple trend, providing strong evidence that the climate system is exhibiting complex system behaviour on decadal timescales.

This model indicates that in situ warming of the atmosphere does not occur; instead, a store-and-release mechanism from the ocean to the atmosphere is proposed. It is physically plausible and theoretically sound. The presence of step-like – rather than gradual – warming is important information for characterising and managing future climate risk.

27 thoughts on “Scientists explore causes of the pause in warming, perhaps the most important research of the decade”

It may be worth changing “Today we look at papers exploring the cause in warming of the surface atmosphere temperature, now in its second decade.” in the summary to “Today we look at papers exploring the pause in warming of the surface atmosphere temperature, now in its second decade.”

I understand your point, comparing America’s political spectrum to the far wider political range of our peers (e.g., in Europe). However, Right and Left represent the two sides of the political spectrum. There are always two sides to a line.

My apologies. I didn’t notice that you were Stephen Wilde — of the Stephen Wilde theory JoNova described. Personally I only follow research published by peer-reviewed journals and climate agencies — and don’t have the time do that (my copies of Nature and Science pile up). Other works enter the climate science debate, such as those published by the Global Warming Policy Foundation. However, I leave this “grey literature” for the pros to sort out.

Thank you for posting here about your theory. Best of luck with your work!

Thanks, no problem. I’m all too aware of the disadvantages of my amateur status but after 60 years of intense interest in climate and weather I do have some advantages over those who came to it more recently and focus on too narrow a field within the relevant science.

I really do recommend that you and others carefully consider the hypothesis that I have put forward since it has been built around observations over half a century rather than on the assumptions relied upon by the climate models.

If my hypothesis is flawed then let someone say so. It has been floating around since 2010 and has not yet been countered by logic or events.

I well understand your problem. We face the same challenge here, publishing non-consensus articles which time has shown to have considerable predictive power. But that doesn’t give us a mass audience. In geopolitics the audience goes to neocons, who have a track record of being consistently wrong.

All in all the pause looks less significant from the vantage point of March 2017. NASA notes 2016 as the hottest year since records started being taken.https://climate.nasa.gov/news/2465/2016-climate-trends-continue-to-break-records/
(note the fluctuations in arctic ice and be aware of the methane stores in the permafrost).
In Australia large parts of the country had record heatwaves.
Now while CO2 might not be the only contributor to the effect, one thing can be pretty well ruled out- and that is solar irradiance, which has, on average been declining since about 1960, with the latest solar maximum being the smallest since about 1930.
Given that solar irradiance is out of the picture that really means that the largest possible non human activity related variable is also removed from consideration.
A serious increase in temperature across the globe carries a very high risk of reducing our ability to feeed ourselves- and carries the risk of serious famines.
In this context (and given the related problems with pollution and species extinction,caused by overpopulation) the case for both encouraging a smaller environmental footprint (including CO2) per person, and a preference for smaller family size really the only humane choices we have to see our grandchildren do not face a bleak future.

“NASA notes 2016 as the hottest year since records started being taken.”

That’s a headline, but NASA’s story is more complicated. The 2016 temperature anomaly was a record high: 0.07°F warmer than 2015 and 0.4°F warmer than the previous El Nino peak in 1998 (0.04 by the satellite data). But the actual 2016 anomaly was 1.69°F ± 0.27°F — so the increase over previous records was not statistically significant. Details here.

“A serious increase in temperature across the globe carries a very high risk of reducing our ability to feeed ourselves- and carries the risk of serious famines.”

I suggest you read about the RCP scenarios provided by the IPCC in the Working Group I report in AR5, and the work built upon them. They give confidence estimates for trends. “Very high” is a bit of an exaggeration.

“and a preference for smaller family size really the only humane choices we have to see our grandchildren do not face a bleak future.”

I suggest reading about population trends. Fertility has collapsed far below replacement level in most of the developed nations, and is falling even faster in most developing nations.

The climate trends of the next 50 years probably depend more on economic growth rates — and even more on the rate of tech progress. The result: RCP8, the worst case scenario (driving the articles you appear to have read), appears increasingly unlikely with every passing year. For details see this.